1. Field of the Invention
The present invention relates to fixing apparatuses of image forming apparatuses in which a color processing mode or a monochrome processing mode in printing process is selectable.
2. Related Art
In recent years, the print processing speeds of image forming apparatuses have been becoming faster. Furthermore, in addition to conventional monochrome printing image forming apparatuses, development of color printing image forming apparatuses has been advancing. As an example of color printing image forming apparatuses there are image forming apparatuses that use an intermediate transfer system in which a color image is formed on a sheet of paper by using a plurality of electrostatic latent image carriers with color separated image information of a plurality of colors (for example, four colors of K, C, M, and Y) and overlaying and transferring these onto an intermediate transfer belt that rotates and contacts the electrostatic latent image carriers with a predetermined pressure, after which transfer of the images as a combined whole is carried out onto a sheet (sheet of paper) that is transported from a paper feed portion (paper feed cassette for example).
In the above-described image forming apparatus, there is a gap in the number of print processing sheets between color processing mode and monochrome processing mode, and it is common for monochrome processing mode to have processing speeds generally 2 to 3 times faster compared to color processing mode.
In the fixing apparatus installed in this image forming apparatus, the power supplied to the heaters of the heat roller and the pressure roller is determined based on the total power consumption determined for the image forming apparatus.
However, in an image forming apparatus that has processing modes in which the number of sheets being passed varies, it is difficult to achieve appropriate fixing performance corresponding to each processing mode when the power consumption of the fixing apparatus is constant. That is, although it also depends on power consumption, when the power consumption is set so as to obtain an appropriate fixing performance in a slow processing mode, there is the possibility that fixing defects will be produced in the fast processing mode, and conversely, when the power consumption is set so as to obtain an appropriate fixing performance in the fast processing mode, there is the possibility that excessive fixing will occur in the slow processing mode. In attempting to deal with such fixing defects and excessive fixing without varying the power consumption, it is conceivable to change the pressing force of the heat roller and the pressure roller according to the processing mode.
In this case, a fixing apparatus described in JP H8-6426A is proposed as a mechanism for adjusting the pressing force of the heat roller and the pressure roller. With this fixing apparatus, a journal bearing is mounted on a pressure lever and the pressure roller can slide vertically inside a slit provided in a lower frame. Furthermore, the pressure lever can swing being pivotably mounted on a shaft and by applying a pressure spring on the end thereof, the pressure roller is caused to press against the fixing roller. Furthermore, two shaft bearing holes are provided for the pressure lever and by varying the bearing hole into which the shaft is inserted, a distance between the shaft (fulcrum of the pressure lever) and the point of application of the pressure spring is changed, which enables the pressing force to be varied.
With the fixing apparatus described in JP H8-6426 A, although the pressing force of the heat roller and the pressure roller can be adjusted in such ways as varying the shaft bearing holes into which the shaft is inserted and by applying an actual pressure spring, these adjustments involve merely adjusting the pressing force of the heat roller and the pressure roller to an always-constant pressure, and since the adjustments are manual adjustments, this does not involve adjusting the pressing force of the heat roller and the pressure roller according to the processing mode.